The present invention relates to a cooling system for solid state power devices and in particular to heat sink apparatus for power semi-conductor devices.
Semi-conductor power elements, for example, the silicon type, produce large quantities of heat during their operation and the problem of safely dissipating this heat is of critical concern. In general semi-conductor power elements are sandwiched between opposing rigid heat sinks of suitable material having good thermal conductivity, such as for example, copper or aluminum members. The semi-conductor, commonly a wafer, is sandwiched between a pair of such members which are clamped together by external spring means so as to apply a high degree of pressure evenly over the entire surface area of the wafer. This pressurized contact attempts to achieve and maintain low electrical and thermal contact resistance and acts to conduct heat away from the semi-conductor. The heat sink formed by the copper or aluminum member is usually provided with cooling fins which enables enlargement of the area cooled and an enlarged radiating surface over which the flow of air may be provided.
It has also been well known to provide heat sinks with a plurality of cooling fluid ducts disposed immediately adjacent to the surface of the semi-conductor wafer sandwiched between the heat sink members. In this way a liquid coolant passing through the ducts may be effectively used to cool the wafer.
The heat sinks of the kind heretofore described usually occupy a large amount of space and employ rather critical material. Accordingly, they are relatively expensive to manufacture and are large and heavy in use. In those heat sinks employing liquid coolant a difficulty arises from the necessity of electrically insulating the liquid coolant medium from the semi-conductor wafer, because of the high operating voltages placed on the semi-conductor itself.
Recently an entirely new technique has been involved for cooling semi-conductor elements employing the principle of a heat pipe. A mass flow of liquid is achieved inside a closed container by means of capillary action, the heat being transferred from one point to another in the form of latent heat of vaporization. The containers holding the mass of liquid is usually spaced in a housing body forming the heat sink which body is in pressure contact with the flat surface of a wafer packaged semi-conductor device. The known heat pipe cooling system suffer from the disadvantage that the capillary structure is disposed over the prevailing or entire internal area of the container. This causes a significant decrease in the transfer of thermal energy. Another limiting factor exisits in that the complexity of the shape of the container holding the liquid precludes the use of the most suitable porous materials.
It is an object of the present invention to provide an improved cooling system for semi-conductor devices which overcome the disadvantages and defects of the known prior art.
It is another object of the present invention to provide an improved cooling system and/or heat sink for wafer type semi-conductor devices which employes a closed evaporating-condensing cycle for transporting heat from the heat source to the heat sink.
It is still another object of the present invention to employ an improved cooling system for semi-conductor devices employing a capillary structure for the return of the condensate in the evaporating-condensing cycle.
It is a further object of the present invention to provide an improved cooling system operating on a closed evaporating-condensing cycle which is simple in construction, inexpensive and light in weight.
The foregoing objects, together with other objects and numerous advantages will be apparent from the following disclosure of the preferred form of the present invention.